First slide

Enthalpy of formation

Question

The enthalpy of elements in their standard states is taken as zero. Hence the enthalpy of formation of a compound is

Moderate

A

always negative
B

always positive
C

positive (or) negative
D

equal to zero

Solution

Consider following example ;
$C (s) + O_{2} (g) \to {CO}_{2} (g), ∆ H = - 393 kJ, ∆ H = H_{P}^{0} - H_{R}^{0}; Since H_{R}^{0} in this case is zero, ∆ H = H_{P}^{0} = - 393 kJ; Similarly for an endothermic process, ∆ H = H_{P}^{0} = + ve;$

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